Treated bottom ash medium and method of arsenic removal from drinking water

ABSTRACT

A method for low-cost arsenic removal from drinking water using chemically prepared bottom ash pre-treated with ferrous sulfate and then sodium hydroxide. Deposits on the surface of particles of bottom ash form of activated iron adsorbent with a high affinity for arsenic. In laboratory tests, a miniscule 5 grams of pre-treated bottom ash was sufficient to remove the arsenic from 2 liters of 2400 ppb (parts per billion) arsenic-laden water to a level below 50 ppb (the present United States Environmental Protection Agency limit). By increasing the amount of pre-treated bottom ash, even lower levels of post-treatment arsenic are expected. It is further expected that this invention supplies a very low-cost solution to arsenic poisoning for large population segments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority from U.S. provisional patentapplication Ser. No. 60/550,668 filed Mar. 4, 2004, entitled “ArsenicRemoval from Drinking Water Using Treated Bottom Ash”, and United Statesprovisional patent application filed Feb. 14, 2005, entitled “ArsenicRemoval from Drinking Water Using Treated Bottom Ash”, both of which arehereby incorporated by reference in their entireties.

STATEMENT REGARDING FEDERAL FUNDING

This invention was made with U.S. Government support under ContractNumber DE-AC03-76SF00098 between the U.S. Department of Energy and TheRegents of the University of California for the management and operationof the Lawrence Berkeley National Laboratory. The U.S. Government hascertain rights in this invention.

REFERENCE TO A COMPUTER PROGRAM

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to arsenic removal from drinkingwater, and more specifically relates to arsenic removal from drinkingwater using treated bottom ash.

2. Description of the Relevant Art

U.S. Pat. No. 6,042,731, by Dagmar Bonnin, entitled “Method of RemovingArsenic Species from an Aqueous Medium Using Modified Zeolite Material”,incorporated herein by reference, discloses a method for treatment ofarsenic-laden drinking water using zeolites. However, zeolites areexpensive, and not economically viable for populations living inpoverty. The patent also discloses the use of FeSO₄-treated fly ash forremoval, however, fly ash is an important, and hence expensive,industrial commodity used in a variety of chemical processes, e.g., inthe manufacture of cement. The treatment of bottom ash is not disclosedin said patent, nor is pretreatment with ultraviolet light exposure tomodify the aqueous Arsenic to a higher valence state As³⁺ to As⁵⁺ so asto increase adsorption of the specie.

BRIEF SUMMARY OF THE INVENTION

This invention provides for a method for arsenic impurity removal fromdrinking water, the method comprising: a) collecting bottom ash; b)treating said bottom ash with FeSO₄ solution; and c) then treating saidbottom ash with NaOH solution. The initial collected bottom ash may befirst washed to remove impurities. Also, said bottom ash may have afurther step of further washing the bottom ash after treatment with theNaOH solution.

The treated bottom ash above may further comprise the step of oxidizingsaid bottom ash. The oxidizing step may be performed by air drying,addition of hydrogen peroxide, potassium permanganate (KMnO₄), bubbledair, sodium hypophosphite (NaH₂PO₂.H₂O), hydrogen peroxide (H₂O₂), orother similar oxidizing agent. Additionally, nonoxidizing agents such asFeSO₄ may provide a color center for absorption of ultraviolet light.Such absorbed photons provide energy for the surface oxidation of thecoated bottom ash from an Fe(OH)₂ state to an Fe(OH)₃ state in theaqueous solution. These states are readily discernable by the yellowishto greenish patina of the Fe(OH)₂ compared to the rust colored state ofFe(OH)₃. When the treated bottom ash is coated with the Fe(OH)₃, it isready to act as an adsorbent material for impurities in a water source,such as arsenic or possibly other heavy metals.

After said treated bottom ash has been prepared and oxidized, one maybegin removing arsenic impurities from a quantity of water using saidtreated, dried bottom ash. The removing step is typically performed byadsorbing arsenic ions from a quantity of water in a form complexatedwith iron ions previously coated on the treated bottom ash surface.

The input quantity of water comprising aqueous arsenic ions may beenhanced by exposing said quantity of water to ultraviolet radiationsufficient to change said aqueous arsenic to a higher valence state,preferably in the presence of dissolved oxygen. The source of thisoxygen is preferably from the atmosphere as in air drying, but can alsobe from bubbled air, or addition of hydrogen peroxide, potassiumpermanganate (KMnO₄), or sodium hypophosphite (NaH₂PO₂.H₂O). In thismanner the valence level of arsenic may raised from As³⁺ to As⁵⁺. It isthought that the As⁵⁺ valence state is more easily adsorbed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings, which are for illustrative purposes only:

FIG. 1 is a flow chart of the pre-treatment and use steps of bottom ashfor aqueous arsenic removal.

FIG. 2 is a chart of residual arsenic remaining after successive batchesof 2400 parts per billion (ppb) arsenic-laden drinking water has beentreated with the adsorbent treated bottom ash of this invention.

FIG. 3 is an electron micrograph of untreated bottom ash, showing asmooth bottom ash particle surface with a mixture of sizes from lessthan 1 μm to 10 μm diameter.

FIG. 4 is an electron micrograph of coated bottom ash, showing theFe(OH)₃ coated bottom ash with a smooth to flaky surface appearance onparticles of all sizes ranging from less than 1 μm to about 10 μmdiameter, covering substantially all of the particulate surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Definitions

Bottom ash means the refuse left behind after partial or completecombustion of bituminous, anthracitic, or lignitic coal or wood, rice,rice husks, or other partially or completely combusted carbonaceousmaterial.

Fly ash means “fine particulate, essentially noncombustible refuse,carried in a gas stream from a furnace”, as defined in the McGraw-HillDictionary of Scientific and Technical Terms, Fifth Edition,McGraw-Hill, Inc., 1994.

Introduction

Arsenic poisoning is a painful, disfiguring, and permanent diseaseafflicting many millions of people world-wide. The invention describedherein takes commonly available coal combustion byproduct ash, or bottomash, pre-treats it with two relatively simple chemical process steps,and results with an adsorbent pre-treated bottom ash that is useful forremoving arsenic and possibly other harmful heavy metals from drinkingwater. Since most of the bottom ash material occurs readily where peopleinhabit, treatment systems using this method have low transportationcosts. Additionally, since processing costs are minimized, overalltreatment costs are very low, and believed readily accessible tocountries which have even very low per-capita income levels.

An example below details a laboratory procedure for preparation of thecoated bottom ash.

EXAMPLE 1 Preparation of 5 Grams of Pre-Treated Bottom Ash

Initially, 5 grams of dry bottom ash are placed in a holding container.

Approximately 30 ml of 0.6 Molar FeSO₄ is mixed with the bottom ash, andcontinuously stirred for an hour at near room temperature, e.g. 10-40degrees C. Although this is the room temperature saturationconcentration of the FeSO₄, lower concentrations could be used foreither lower bottom ash surface coverage, or lower quantities of bottomash. It is possible that FeSO₄ concentrations could be usable from therange of 0.1 M or greater.

After stirring, the solid bottom ash particulate is allowed to settle tothe bottom of the holding container for about 5 minutes. At this point,the surface liquid is removed, filtered, and the residue on the filterpaper added to the dense mixture at the bottom of the holding container.

The holding container is then stirred with an aqueous solution of 9 mlof 0.5 N NaOH, and the contents mixed well for 5 minutes. It is thoughtthat the NaOH processes the previous FeSO₄ into a complex ferrous stateof Fe(OH)₂ according to the oxidation reduction chemical formulaFeSO₄+2NaOH→Fe(OH)₂+Na₂SO₄, which forms an improved arsenic adsorbent.

Drain off the excess surface liquid after the solids are allowed tosettle to the bottom of the holding container for about 5 minutes. Atthis point, the surface liquid is removed, leaving a dense processedmixture at the bottom of the holding container.

Spread the contents of the holding container on a filter paper, place ina Petri dish, and dry at room temperature in open air in a fume hood fora period of 36 hours. This serves the function of oxidizing the coatedFe(OH)₂ bottom ash to form a coated Fe(OH)₃ bottom ash, which has beenfound to be much better at adsorbing arsenic in water than the Fe(OH)₃form.

Wash the dried material from the previous step three times using eachtime about 100 ml arsenic-free water to remove any non-adsorbed NaOH orFe(OH)₂ solute. At the end of each wash, filter the supernatant, and addthe residue on the filter paper back to the solids. At the end of thethird wash, the solids are the prepared medium (adsorbent).

Use the adsorbent media in the following manner: add arsenic-laden waterto the adsorbent, stir for a sufficient time to adsorb aqueous arsenic(typically about an hour), and then decant the liquid.

In laboratory test, the adsorbent medium was able to remove arsenic from2000 ml of 2.4 ppm (parts per million, or alternatively 2400 parts perbillion, ppb) arsenic laden water to bring the arsenic concentration toa level of 50 ppb or below, as described in FIG. 2.

Applied Use of the Pre-Treated Bottom Ash for Arsenic Removal

In practice, an arsenic laden water source is mixed with the coatedFe(OH)₃ coated bottom ash sufficiently to reduce the arsenic levels toan acceptable level. Presently, for the United States EnvironmentalProtection Agency (US EPA), that level is 50 ppb, however, it will bereduced to 10 ppb beginning January 2006. The World Health Organization(WHO) has already announced that it recommends the provisional value foracceptable arsenic concentration in drinking water be changed to a lowerlimit to below 10 ppb.

Improved adsorption performance may be obtained by pre-treating thearsenic laden water with ultraviolet light, so as to change the state ofthe aqueous arsenic from As³⁺→As⁵⁺. Normally, clear water allowsultraviolet light to pass through without absorption. However, with theaddition of a color center source, such as FeSO₄, ultraviolet light willbe absorbed, energizing the ions to change the arsenic valence state.

It should be noted that atmospheric, freely flowing water, usuallycontains an abundance of As⁵⁺ due to the oxygen present in the water.However, well-borne water fails to have an oxygen source, hence ispredominated by As³⁺ state. It is also believed that the state change tothe pentavalent state can be achieved through oxidation additives suchas bubbled air, potassium permanganate (KMnO₄), sodium hypophosphite(NaH₂PO₂.H₂O), or hydrogen peroxide (H₂O₂). Such additives may be dry oraqueous based.

As with both of the two water sources above, arsenic is removed thoughadsorption of the arsenic ions onto the pre-treated bottom ash. At somepoint, most receptor sites are occupied by arsenic ions, precludingreaching the maximum acceptable level. At this point, the pre-treatedbottom ash is disposed, and replaced with new pre-treated bottom ash.

It has been found experimentally that large oversupplies of coatedFe(OH)₃ bottom ash (amounts sufficient to normally treat 25 times moreof As⁵⁺) have successfully removed water borne arsenic to acceptablelevels of 10 ppb or lower, even without raising the arsenic valencestate to As⁵⁺. Thus, it is possible to calculate whether it is moreeconomical to use more media, or raise the arsenic valence level to thepentavalent state.

Refer now to FIG. 1, which shows the process flow for treatment ofbottom ash to manufacture coated Fe(OH)₃ bottom ash. These steps followthose of Example 1 described above.

Refer now to FIG. 2. To give a sense of scale of the effectiveness ofthe coated Fe(OH)₃ bottom ash media, highly arsenic laden water havinginitial concentrations of 2400 ppb (or 240 times the proposed WHOlimits) is sequentially input, and after mixing, tested for arsenicconcentrations. Using only 5 g of coated Fe(OH)₃ bottom ash, the initial2400 ppb water only exceeds the 10 ppb limit at a total treated volumeof 1500 mL. It appears logical that a sequentially staged treatmentsystem would bring the output arsenic levels to extremely low levelswell below the proposed 10 ppb level.

Refer now to FIG. 3, which shows uncoated bottom ash in an electronmicrograph. The bottom ash particles vary in size, and tend to have asmooth surface.

Refer now to FIG. 4, which shows Fe(OH)₃ coated bottom ash. The natureof the coating is such that it appears to have one or more layers, andsubstantially covers each particle, regardless of size.

It should be noted that all of the discussions before have mixed theFe(OH)₃ coated bottom ash in the arsenic laden water for arsenicremoval. Alternative implementations could include a filter bag, withthe arsenic laden water dripped though, or a percolation bed that hasthe water to be filtered passing through the bed. Additionally, any ofthese systems could be mixed or matched to provide for staged recoveryof arsenic. Perhaps the easiest method is to mix the Fe(OH)₃ coatedbottom ash with the arsenic laden water for a period of time by simpleair bubbling. After ceasing air bubbling, the arsenic laden Fe(OH)₃coated bottom ash should simply settle to the bottom, allowing waterwithout arsenic to be withdrawn from the top.

CONCLUSIONS

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication or patent application were eachspecifically and individually indicated to be incorporated by reference.

The description given here, and best modes of operation of theinvention, are not intended to limit the scope of the invention. Manymodifications, alternative constructions, and equivalents may beemployed without departing from the scope and spirit of the invention.

1. A method for coating bottom ash for arsenic removal in water,comprising: a) mixing a quantity of bottom ash in a liquid comprisingwater to form a bottom ash suspension; b) treating said bottom ashsuspension with an FeSO₄ aqueous solution to form a treated bottom ashsuspension; and c) then reacting said treated bottom ash suspension witha NaOH aqueous solution to form a coated Fe(OH)₂ bottom ash suspension.2. The method of claim 1 wherein said treating step FeSO₄ aqueoussolution comprises: a) FeSO₄ solution greater than 0.1 M.
 3. The methodof claim 1 wherein said treating step FeSO₄ aqueous solution comprises:a) saturated FeSO₄ solution of about 0.6 M.
 4. The method of claim 1wherein said reacting step NaOH aqueous solution comprises: a) NaOHsolution at about 0.5 N.
 5. The method of claim 1 further comprising: a)means for adsorbing aqueous arsenic from a quantity of water using saidbottom ash.
 6. The method of claim 5 further comprising the step of: a)means for changing said aqueous arsenic at As³⁺ to a higher valencestate of As⁵⁺.
 7. The method of claim 1 comprising: a) oxidizing saidcoated Fe(OH)₂ bottom ash suspension to form a coated Fe(OH)₃ bottom ashsuspension.
 8. A method of aqueous arsenic removal comprising: a)contacting an aqueous arsenic source with the coated Fe(OH)₃ bottom ashsuspension of claim
 7. 9. A coated Fe(OH)₃ bottom ash solutioncomprising: a) the coated Fe(OH)₃ bottom ash suspension of claim
 7. 10.The method of claim 7 wherein said oxidizing step comprises: a)oxidizing with potassium permanganate (KMnO₄).
 11. The method of claim 7wherein said oxidizing step comprises: a) oxidizing with bubbled air.12. The method of claim 7 wherein said oxidizing step comprises: a)oxidizing with sodium hypophosphite (NaH₂PO₂.H₂O).
 13. The method ofclaim 7 wherein said oxidizing step comprises: a) oxidizing withhydrogen peroxide (H₂O₂).
 14. The method of claim 1 comprising: a)drying said coated Fe(OH)₂ bottom ash suspension in the presence ofoxygen to form a dried coated Fe(OH)₃ bottom ash.
 15. A dried Fe(OH)₃coated bottom ash comprising: a) the dried coated Fe(OH)₃ bottom ash ofclaim
 14. 16. The method of claim 1 comprising: a) washing the coatedFe(OH)₂ bottom ash suspension to substantially remove the NaOH solution;b) providing a color center source; and c) exposing the coated Fe(OH)₂bottom ash suspension with the color center source to an ultravioletradiation to oxidize the coated Fe(OH)₂ bottom ash suspension into acoated Fe(OH)₃ bottom ash suspension.
 17. The method of claim 1 whereinthe color center source comprises: a) an FeSO₄ aqueous solution.
 18. Amethod for treating bottom ash for arsenic removal in water, comprising:a) providing a quantity of bottom ash; b) means for forming a coatedFe(OH)₃ bottom ash in suspension from said quantity of bottom ash. 19.The method for treating bottom ash for arsenic removal in water of claim18 further comprising: a) means for contacting an aqueous arsenic sourceto said coated Fe(OH)₃ bottom ash in suspension.
 20. A Fe(OH)₃ coatedbottom ash particle comprising: a) a bottom ash particle having asurface, comprising: i) a layer of Fe(OH)₃ on the bottom ash particlesurface, ii) whereby said layer of Fe(OH)₃ forms a Fe(OH)₃ coated bottomash particle.
 21. An aqueous arsenic removal medium comprising: a) theFe(OH)₃ coated bottom ash particle of claim 20 contained in a medium.